In paper and paperboard manufacture, fillers are added to increase the brightness and opacity of paper, as well as for other advantages such as improved smoothness, printability, and lower material costs. Fillers are fine particles of insoluble mineral material which are impregnated in and among the paper pulp fibers. Alkaline fillers, such as calcium carbonate, are commonly used due to the prevalence of alkaline process conditions in paper manufacture. It has become desirable to load paper with as much filler as possible to obtain the desired properties without degrading the strength of the paper by interfering with the bonding between fibers.
Filler is added to paper conventionally by mixing or precipitating filler material in a paper pulp slurry so that it is retained in the pores and spaces between and among the paper fibers. However, filler addition to the pulp slurry leads to waste of large amounts of filler material as residue and requires further processing to remove the filler material from waste water in the papermaking process. Other techniques, for example, as disclosed in U.S. Pat. No. 5,122,230 to Nakajima and U.S. Pat. No. 5,233,090 to Klungness et al., have provided for in situ attachment of filler in the pulp by using the hydrophilic properties of virgin cellulosic fibrils to take up a salt-containing solution, then contacting the salt-laden pulp fibers with a gas or aqueous agent to precipitate the filler particles on the surface of or within the fibers.
For environmental reasons and the reduction of solid waste, increasing amounts of paper are being recycled today. The recycling of paper requires repulping fiber from waste paper, referred to in the industry as secondary fiber pulp. Secondary fiber pulp may be deinked or undeinked. Deinked pulp is produced by any one of the known methods for contaminant and ink removal with or without bleaching. However, it is also common to use secondary fiber pulp that has not been deinked for the manufacture of some types of paper.
Heretofore, filler has been added to secondary fiber pulp during papermaking. As explained above, filler addition to the pulp slurry leaves large amounts of the filler material as residue and requires further processing for its removal. Secondary fibers when repulped are different mechanically and chemically from virgin fibers because they have charged species, ink, surface-active agents, etc., that can interfere with the deposition of filler precipitate on the fibers. Secondary fibers also have different bonding characteristics and may be hydrophobic or hydrophilic, compared to virgin fibers which are hydrophilic. Thus, in situ attachment of filler precipitate has not been used for secondary fibers.